Coupled network



June 4, 1946. R. B. HOFFMAN 2,401,353

COUPLED NETWORK Filed July 21, 1942 SOUPCE /2 H/gH FEEQUf/l/C) vEXC/T/NG .S/GIVAL INPUT 4:, I

INVENTOR 055 a l/OFFMAA/ Patented June '4, 1946 COUPLED NETWORK Ross B.Hoffman, East Orange, N. J., assignor to Federal Telephone and RadioCorporation, a

corporation of Delaware Application July 21, 1942, Serial No. 451,721

1 18 Claims.

This invention relates to transmission lines and more particularly to ahigh frequency controlling network coupled thereto.

In my copending application, Serial No. 418,671 filed November 12, 1941,I disclosed antenna controlling network sections by which concurrent andalternate transmission and reception over a common antenna was madepossible. As therein disclosed, I accomplished this by controlling theresonance of quarter wavelength network sections coupled to thetransmission lines connecting a common antenna to a transmitter and areceiver. It is known that a, resonating network section one quarterwavelength long, short circuited at one end, and coupled to a.transmission line may introduce substantial voltages at the resonantfrequency into the line in the zone of the coupling. If the resistanceof the coupled section is negligible, these voltages-may be so large aseffectively to block or prevent the passage of power along thetransmission line beyond the point where the section is coupled. In myaforesaid application, I utilized this blocking efiect of the quarterwave short circuited coupled sections to effect a switching of theantenna at will from the transmitter circuit to the receiving circuitand vice versa.

This switching was effected by using a control circuit for. two couplednetwork sections, one for each circuit, whereby the shunt reactancebetween the open parts of each of the coupled sections could be varied,thus changing the operation of the sections at will between resonanceand non-resonance. One of the means for controlling the resonant andnon-resonant operation of thesecti'ons included'a vacuum tube circult inwhich the output of the vacuum tube was applied or removed from acrossthe open part of each section. By this means, the shunt impedance ofeach section was changed so that when the tube was at cut-oil, thesection could be in resonance with the transmission line in one form ofthe aforesaid invention and when a certain signal or impulse was appliedto the grid of the tube, an effective shunt impedance was establishedthereby detuning or in other words producing a non-resonant condition ofthe section.

- "l fhe present invention is an improvement of the" coupled controllingnetwork feature of the invention covered in my aforesaid application,

I and one of the objects of the present invention 2 ditions of which maybe controlled by variations in very small voltages.

Another object of the invention is to provide a coupled networkcontrollable by a small reactance tube as compared to high power tubesrequired for controlling the coupled sections heretofore proposed.

Still another object of the invention is to provide a network havingvacuum tube control of the tuning thereof wherein the network is'of sucha character that effective tuning and/or detuning of the network ispossible with but a small tube reactance swing.

A further object of the invention is to provide an improved method forcontrolling the resonant condition of a network coupled section.

The above objects and others ancillary thereto are accomplished by myinvention by usinga doubled coupled network of sections. ferred form,one of the sections is adapted to resonate at a particular wavefrequency and the second of the sections, which is slightly dissimilarto the first-mentioned section, is adapted to be tuned to the resonancefrequency thereof and thereby alter the resonating condition of thefirst section. This second section of the network is coupled with thefirst section and the length thereof is such that when its impedance ischanged to a value corresponding to the impedance of the first section,it tends to resonate therewith thereby altering the impedance of thefirst section. This adverse effect to the resonance condition ofthe-first section with respect to the transmission line with which itmay be coupled reduces the resonant voltages developed in the linethereby unblocking the line to wave transmission at the aforementionedfrequency. This variation of the impedance of the second section, so asto bring the impedance thereof in line with the impedanceof the firstsection, may be made by the use of a vacuum tube circuit in which thevariation of impedance may be ef-- fected by variation of the gridvoltage of the tube. This methcxi of controlling the resonance andnon-resonance of the first section by means of a second section is ofgreat advantage over the application of a tube circuit directly to thevfirst section in that the controlof the resonant condi-' tion can beeil'ected by a vacuum tube having a much smaller reactance. It alsopermits connection of the tube circuit to-the section further out towardthe open end thereof thereby greatly increasing the shunt capacityeffect of the tube circuit upon the impedance of the section.

The coupled relation ofthe two network sec- In a pre- For anunderstanding of these several relationships of the sections and otherfeatures of the invention, reference may be had to the followingdetailed description to be read in connection with v the accompanyingdrawing in which the sole figure thereof represents schematically atypical form of apparatus by which the aforementioned relations and themethod of control of this invention may be performed.

Referring to the drawing, a source It or high frequency alternatingcurrent, such as a. transmitter, is shown for purposes of illustratingthe invention, connected by a transmission line ll to a load 12, whichmay be an antenna. It will be understood, oi. course, that thisinvention may be used with an antenna which supplies the high frequencycurrent and the part ID may comprise a receiver. I I Coupled to thetransmission line I4 is a right angle halt wave network l8, one leg A ofwhich comprises a pair of conductors I8 and I! of the order of orapproximately a quarter wavelength and short circuited at one end by ashort circuiting conductor l8.- This leg of the network is arrangedparallel to the transmission line ll. The other leg B of the networkcomprises a pair or conductors and 2! connected at one end to the openends or the conductors I8 and i1 and short circuited at the other end bya short circuiting conductor 22. The two legs, each being in the orderof a quarter wavelength, are adapted to resonate with the transmissionlines when the transmission lines carry power having the wavelength forwhich the network coupling is designed. The leg A is parallel to theline H While the leg B is at right angles thereto. At resonance, thevoltages built up in the leg A at its end or junction IS with leg B maybe such that the voltages effectively block the passage of the power atthe point where the network is coupled to the transmission line.

In accordance with my invention, 1 provide a second network 25comprising parallel conductors 26 and 21 arranged p rallel to theconductors 20 and 2| of the leg B. A short circuiting conductor 28 isprovided to short circuit the conductors 28 and 21 at one end of thesection. Thesection 25, for the preferred relationship is of anelectrical length slightly shorter than a quarter wavelength so that itwill not resonate with the network it unless the electrical length ofthe section 2| is changed by resonance exciting energy to, correspond tothe electrical length of the section ii. I accomplish this by providinga vacuum tube circuit along the lines or either of the embodimentsdisclosed in my oopending application Serial No. 418,671. For purposes0! illustratio'n, I have chosen a single vacuum tube circuit 30. Bymeans ot-the vacuum tube circuit, the resonant condition or the sectionII is controlled by controlling the resonance excitin energy applied tothe section 2|. This control may be eflected by ignals or other impulse;re-

ceived at the input 43 of the vacuum tube circuit. The resonanceexciting signal input may be applied in any well-known manner such asover a coupling condenser 4| and across a resistance which in the iorm,shown includes a resistor 31 and aportion of a bias-controlpotentiometer 32.

The vacuum tube circuit 30 includes a tube 3|, 3

the output 01 which may include the usual cathode, control grid, secondgrid, and anode elements, and may be applied to the section 25 at points32 and 33, the voltages at these points being 180 out of phase. Thesupply lines to the points 32 and 33 each include a condenser 34 and 35respectively. Connected to the condenser 34 is a resistor 36 which ispreferably of such a magnitude that the value of the R. C. networkformed by the resistor 36 and the grid cathode capacitance of tube 3iwill displace the phase of the high frequency grid voltage substantiallywith respect to that at the point 32. Inasmuch as the plate current ofthe tube 3| is in phase with the voltage applied to the control grid,this current leads the voltage at point 33 because voltages at point 32and 33 are out of phase. It thus becomes clear that the plate currentfed into section 25 through the condenser 35 may produce an eiiectiveshunt capacity across the section since the voltage and current thereofare substantially 90 out of phase.

Since this effective shunt capacity may vary with the amplification oroutput of the tube, an appropriate adjustment of the grid bias by meansof the potentiometer 38 may control the magnitude of shunt reactanceacross points 32 and 33 to a desired degree depending on the magnitudeof blocking signal applied at the input 40. As desired, high frequencychoke 42 and 43 may be provided in the input and output circuits of thetube 3| to isolze the grid and plate supply respectively, and screengrid voltage may then be supplied over a resistor 46, the screen gridbeing connected to ground through a resistor 41.

The control circuit for the section 25 is preferably normally adjustedso that the section will be detuned when no signal or controllingimpulse is received at the input 40. Upon reception of a signal of fixedamplitude. the resultant reactance across the section 25 would be suchas to resonate the section, thus decreasing the flow in the network i5to which the section is coupled.

In operation of the control circuit, assuming that no controlling signalis received at the input 40, the section 25 will have an impedance suchthat it will not resonate with the leg B of the section Hi. The sectionI5 is adapted normally to resonate with the high frequency wavestransmitted along the transmission lines H. The section H5 at resonancedevelops a high potential at the open end or junction 19 of the legs Aand B thereby setting up correspondingly high voltages in the adjacenttransmission lines H, which operate to block and prevent flow of wavesbeyond the coupling therewith of the section l5. Thus, the propagatedwaves of the particular frequency for which the section i5 is designedis normally prevented from reaching the load or antenna i2.

To permit the waves to pass, a signal of small voltage may be applied atthe input 40 thereby establishing by means of the tube circuit 30 ashunt reactance across the section 25 at the points 32 and 33. Thisshunt reactance eil'ect changes the electrical length of the section 25to correspond closely to the electrical length or the leg B of sectionii. The mutual inductance relationship oi the leg B and the section 25under this condition alters the electrical length of the, leg A of thesection IS with respect to the irequency of the current of thetransmission line.

This results in a decrease in the resonance rela- 5. tween the twosections II and 25. The voltages in the section It decrease immediatelyupon the establishment of near resonance between the two sections, andthis decrease limits the building up.

of voltages in the section 25. That is to say,

the voltages built up in the section 25 will be considerably less thanthe voltages in the section l since the voltages of the section l5 aredecreasing as the voltages in section 25 are building up I arerelatively small, it follows that the tube 3|, for control operation,need have only a small reactance value. This is an important advantageover controlling coupled networks heretofore shunt capacity would be inthe nature of a trimming action only. At resonance the voltages acrossthe' open end of the section 25 are maximum, but since this maximum isrelatively low for the section 25 of the network system of my invention,the maximum voltages are not so great as to produce deleterious orundesirable efiects on even a low reactance tube even when theconnections 32 and 33 are located relatively near the open end ofthe-section. The advantage of locating the connecting points 32 and 33more closely adjacent the open end than the short circuit end 28 is thatthe shunt reactance of thetub circuit produces greater effect and,therefore, more sensitive control.

In addition to the above-described resonance controlling relationshipbetween the coupled network sections l5 and 25 of my invention,severalother controlling relationships are contemplated. Assume,- forexample, that the section I5 is normally resonant at the frequency ofthe transmitted current when the section 25 is tuned to resonance at thesame frequency. This resonant relation may e detunedto permit currentflow in the transmission line by applyin an impedance across the section25. For another controlling relationship, assume that the section I! isnormally detuned with respect to the frequency of the transmittedcurrent when the section 28 is tuned. This resonance condition willpermit current fiow. Then when the section 25 is detuned, it has theeffect of tuning the section I! thereby preventing current flow. Forstill another controlling relationship, assume that the section I! isnormally detuned with respect to the frequency of the transmittedcurrent when section 26 is detuned. This condition also permits currentfiow. Then when section 25 is tuned, it will have the efiect of tuningthe section l5 thereby preventing current flow. v Thus, while I haveshown but one embodiment of the invention, I recognize that many changesand variations are possible in the construction and method of controlthereof without departing from the invention. It is to be understood,therefore, that the embodiment as herein shown and described is to beregarded as illustrative of the invention only and not as restrictingthe appended claims.

What I claim is:

ince the voltages produced in the section 25 1. In a high frequencytransmission system having a transmission. line and means for propa- Igating high frequency energy waves therealong; means including a networkto control the transmission of said energy waves along said line, saidnetwork comprising a first transmission line section so coupled to saidline that resonance of. the section at the frequency of said energywaves is operable to substantially block the fiow of said energy wavespast the point of such coupling, a

second transmission line section coupled to the first mentioned section,and means to control the resonance of said first section by controllingthe resonant relationship between said first and said section sections.

2. In a high frequency transmission system having a. transmission lineand means for propagating high ,frequency energy waves therealong; meansincluding a network to control the transmission of said energy-wavesalong said line, said network comprising a firstsection coupled to saidline, said section being resonatable at the frequency of said energywaves to build up voltages operable to substantially block the fiow ofsaid energy waves past the point of such coupling, a second sectioncoupled to the first section. the resonance tuning and detuning of saidsecond section controlling the resonance condition of said 7 firstsection, and means responsive to input signals to apply a. shuntreactance across said second section to control the tuning and detuningof said second section.

3. The coupled network in a high frequency transmission system asdefined in claim 2 wherein said first section is normally resonancetuned at the frequency of the energy waves and said sec- 0nd section isnormally resonance detuned at said frequency and said control means isoperable to resonance tune said second section and thereby diminish theresonance relationship betweensaid resonance detunesaid second sectionand thereby diminish the resonance relationship between said firstsection and said line. n

5. The coupled network in a high frequency transmission system asdefined in claim 2 wherein said first section is normally resonancedetunedat the frequency of the energy waves and said second section isnormally resonance tuned at said frequency and said control means isoperable to resonance detune said second section and thereby tune saidfirst section to resonance at said frequency to block the flow of energywaves along the line. 1

6. In a high frequency transmission system having a transmission lineand means for propagating high frequency energy waves therealong;means'including a network to control the transmission of saidenergywaves along said line, said network comprisinga first sectioncoupled to said line, said section being resonatable at the fre quencyof said energy waves to build up voltages operable to substantiallyblock the fiow of said energy waves past the point of such coupling, apart of said first section being disposed at right angles to said line,a second section coupled to said part, and means to apply a shuntreactance across saidsecond section to control the tuning anddetuningoi'ssid second section to resonance a second section coupled to saidother leg, and

means to apply a shunt impedance to said second section to control thetuning and detuning of said second section to resonance at saidfrequency to thereby control the resonance tuning of said first section.

8. In a high frequency transmission system having a transmission lineand means for propa gating high frequency energy waves therealong; meansincluding a network to control the transmission of said energy wavesalong said line, said network comprising a first section having two legseach in the order of a quarter wavelength,

I other leg being disposed at right angles thereto,

another section coupled to said other leg, and a resonance excitinginput circuit having a tube responsive to input signals to apply a shuntreactance across said second section to control the tuning and detuningof said second section 'to resonance at said frequency to therebycontrol the resonance tuning of said first section.

9. A network for use in controlling the flow of high frequency energywaves along a transmission line comprising a plurality of sections, oneof said sections being of an effective electrical length in the order ofa quarter length of the energy waves-to be controlled, a second of saidsections being coupled with said one section but of a length dissimilarthereto, and said second section having means operable to effect changeof the electrical length thereof to correspond to the electrical lengthof said one section.

10. A network for use in controlling the flow of high frequency energywaves along a transmission line comprising a plurality of sections, oneof said sections having two legs disposed at right angles to each other,each leg being of an electrical length in the order of a quarter lengthof the energy waves to be controlled. a second of said sections beingcoupled with one of said legs, and said second section having aresonance exciting input connection having means responsive to inputsignals to effect change of the electrical length thereof to correspondto the electrical length of said one leg.

11. A network for use in controlling the flow of high frequency energywaves along a transmission line comprising a plurality of sections, oneof said sections having two legs disposed at right angles to each other,each leg being of an effective electrical length in the order of aquarter wavelength of the energy waves to be controlled,

-a second of said sections being coupled with one of said legs, and saidsecond section having means operable to effect change of the electricallength thereof to correspond to the electrical length of said one leg.

12. The method of controlling the flow of high frequency energy wavesalong a transmission line having coupled thereto a network including twotransmission line sections, comprising so coupling one'of said sectionsto said line that resonance of said one section at the frequency of saidenergy waves is operable to substantially block the flow of said energywaves past the point of coupling,

and controlling this resonance relationship of said one section bycontrolling the resonance relationship between said two sections.

13. The method of controlling the flow of high frequency energy wavesalong a transmission line having coupled thereto a network including twotransmission line sections, comprising so coupling one of said sectionsto said line that resonance of said one section at the frequency of saidenergy waves is operable to substantially block the flow of said energywaves past the point of coupling. controlling the other of said sectionsindependently of the flow of said energy waves, and controlling theresonance relationship between said one section and said transmissionline by controlling the resonance relationship between said twosections.

14. The method as defined in claim 13 wherein the controlof theresonance relationship between the two network sections comprisesapplying at will a shunt reactance across the second of the twosections.

15. The method as defined in claim 13 wherein said one section isnormally resonance. tuned at the frequency of the energy waves. thesecond of said two sections is normally resonance detuned at saidfrequency, and the normal resonance condition of said one section isdetuned by tuning said second section to said frequency.

16. The method as defined in claim 13 wherein said one section isnormally resonance detuned at the frequency of the energy waves, thesecond of said two sections is normally resonance tuned at aidfrequency, and the normal resonance condition of said one section isdetuned by detuning said second section.

17. The method as defined in claim 13 wherein the two network sectionsare normally resonance tuned at the frequency of the energy waves, andthe normal resonance condition of said one section thereof is altered bydetuning the second of the two sections.

18. In a high frequency transmission system having a transmission lineand means for propagating high frequency energy waves therealong, meansincluding a network to control the transmission of said energy wavesalong said line, said network comprising a first transmission linesection so coupled to said line that resonance of the section at thefrequency of said energy waves is operable to substantially block theflow of said energy waves past the point of such. coupling, a secondtransmission line section coupled to the first mentioned section, andmeans energized independently of the electrical status of saidtransmission line for controlling the resonance of said second sectionto thereby control the resonance of said first section. 1

- ROSS B. HOFFMAN.

